The present invention relates to a color television cathode-ray tube of the type having three electron beam generating systems, or guns, arranged in a plane, or in line.
In such tubes, the electron beam axes of the three systems intersect at a close distance in front of the phosphor screen and the electron beam generating systems are fastened to insulated bars by means of fastening clamps so that there is a force locking, or positive, fastening of each one of the two outer electron beam generating systems to the center electron beam generating system through the intermediary of at least one insulating bar.
Color picture tubes employing three beam generating systems disposed in a plane are known as in-line tubes.
It has been found during operation of such tubes that with increasing heating of the beam generating systems, color fringes can often be noted on the screen, obviously resulting from the fact that the beam convergence point of the three electron beam generators, which is fixed during assembly of the system so that it lies essentially in the plane of the color shadow mask, is shifted along the longitudinal axis of the tube.
One reason for this undesirable phenomenon is believed to be the fact that the individual electrodes of the beam generating systems expand to a different degree with respect to the electron emitting cathodes, depending on their respective positions, and thus the beam passage openings of the individual electrodes are no longer aligned with one another. This causes the electron beam to be skewed so that the angles at which the electron beams are to impinge with respect to one another are no longer maintained.
Such skewing is particularly noticeable where a high voltage difference exists between two adjacent electrodes so that it is quite possible that even upon a slight shift due to heating, the electron beam may be skewed to a greater extent in the area of the focusing lens where there are generally greater differences in voltage than at a point where the voltage differences are less but the shifts due to heating are greater.